1. Field of the Invention
This invention relates to drilling and casing of wells, and more particularly to a method of inhibiting the initiation and propagation of fractures in susceptible zones as the drilling progresses, and as the casing is installed in the wellbore.
In the drilling of a well into the earth by rotary drilling techniques, conventionally a drilling fluid is circulated from the surface of the earth down a drill string having a drill bit on the lower end thereof and through ports provided in the drill bit to the well bottom and thence back to the surface through the annulus formed about the drill string. Commonly, drilling fluids are employed that are either oil or water based. These fluids are treated to provide desired rheological properties which make the fluids particularly useful in the drilling of wells.
A problem often encountered in the drilling of a well is the loss of unacceptably large amounts of drilling fluid into subterranean formations penetrated by the well. This problem is often referred to generally as "lost circulation," and the formations into which the drilling fluid is lost are often referred to as "lost circulation zones" or "thief zones". Various causes may be responsible for the lost circulation encountered in the drilling of a well. For example, a formation penetrated by the well may exhibit unusually high permeability or may contain fractures or crevices therein. In addition, a formation may simply not be sufficiently competent to support the hydrostatic pressure applied by the drilling fluid and may break down under this hydrostatic pressure and allow the drilling fluid to flow thereinto.
It is this latter situation where the formation is broken down by the hydrostatic pressure of the drilling fluid to which the present invention is addressed. One of the limiting factors in drilling a particular portion of a well is the mud weight (density of the drilling fluid) that can be used. If too high a mud weight is used, fractures are created in lost-circulation zones with resulting loss of drilling fluid and other operating problems. On the other hand, if too low a mud weight is used, encroachment of formation fluids can occur, borehole collapse may occur due to insufficient hydrostatic support, and in extreme cases safety can be compromised due to the possibility of a well blowout. In many cases, wells are drilled through weak or lost-circulation-prone zones prior to reaching a potential producing zone, requiring use of a low mud weight and installation of sequential casing strings to protect weaker zones above a potential producing zone. If a higher weight mud could be used in drilling through weaker or depleted zones, then there is a potential for eliminating one or more casing strings in the well. Elimination of even one casing string from a well provides important savings in time, material and costs of drilling the well.
Thus, there is a need for a method of drilling wells using higher mud weight than could normally be used without encountering formation breakdown problems.
2. The Prior Art
Prior art techniques for dealing with formation breakdown have been remedial in nature, rather than preventive as in the case of the present invention.
Numerous techniques have been developed to control lost circulation. One common technique involves increasing the viscosity of the drilling fluid to increase its resistance to flow into the formation. Another technique involves the addition of bulk material, such as cottonseed hulls, sawdust, or ground walnut shells, to the drilling fluid. In addition, asphaltic materials including gilsonite have been added to both water base and oil base fluids to prevent the loss of drilling fluid from the well into surrounding formations. For example, in U.S. Pat. No. 2,773,670 to Miller, there is taught a method of correcting lost circulation by adding asphaltic material to drilling fluids. These materials are added to the drilling fluid in the form of discrete particles and are suspended as such in the fluid where they are carried down the drill pipe and thence via the annulus between the pipe and the well bore to the lost circulation zone. Upon reaching the lost circulation zone, due in part at least to the elevated temperatures and pressures found in the well, the asphalt particles attach themselves to the formation and are fused or bonded to one another and to the formation structure to form an effective seal against the passage of drilling fluid into the formation.
The use of gilsonite in suitable granular form in an aqueous carrier liquid for correcting lost circulation is taught in U.S. Pat. No. 2,812,161 to Mayhew. The granules of gilsonite are simply mixed with a portion of aqueous drilling mud and pumped into the well bore so that the mud will carry the gilsonite granules to the leaking strata and force them into the crevices therein. In accordance with Mayhew, the effective concentration of the granular gilsonite in the aqueous liquid carrier ranges from as little as 10 to as much as 50 pounds per barrel (ppb) of the mixture for most common cases of lost circulation, and for severe conditions this concentration may be increased to as much as 100 pounds per barrel.
In U.S. Pat. No. 2,573,690 to Cardwell et al. there is described a method of treating earth formations wherein a filler, which is a solid substance in granular form in two ranges of particle size, one being coarse and corresponding in size to the channel fissures and cracks, the other comparatively fine and corresponding in size to the particles of the formation to be consolidated, is used in conjunction with an earth consolidating resin-forming liquid mixture. Among those substances suggested as filler are walnut shells, pecan shells, coconut shells, and gilsonite.
Other granular materials used in the control of loss of fluid into formations include coal and coke. For example, in U.S. Pat. No. 2,650,195 to Cardwell et al., there is disclosed a method of controlling the loss of fluid into formations by providing a cohesive resinous coating upon filterable inert solid particles which are added to the fluid so as to form a filter cake of self-bonding particles when a thief zone is encountered. Various filler materials which are disclosed include coke, coal, cinders, and nut shells. The "coke" which has been used in these prior art techniques has been derived from coal by subjecting coking coal to partial oxidation in coke ovens.
U.S. Pat. No. 3,788,406 to Messenger describes addition of an emulsifying agent and a large amount of ground coal or asphaltic oil-wettable granular particles to control lost circulation.
U.S. Pat. No. 4,957,174 to Whitfill et al. describes a method of correcting lost circulation, after it has been encountered, by adding particulate petroleum coke particles in a specific size range to essentially plug off openings in a thief zone.
U.S. Pat. No. 3,053,764 to Hummel et al. describes a method of controlling lost circulation by addition to the drilling fluid of a mixture of large particles of coke and bentonite.